In recent years, there has been a demand for energy conservation to protect the environment. In the manufacture of battery-driven devices such as mobile phones and digital cameras, increased importance is placed on the reduction of electricity consumed by the battery-driven devices in order to extend a battery life. For this reason, a non-insulative step-down switching regulator including an inductor (hereinafter referred to simply as switching regulator) is widely used as a power supply circuit because of high-efficiency and ability to be downsized. Although the switching regulator is highly efficient when connected to a rated load, the amount of current consumed by the switching regulator is relatively large. Therefore, when the device connected to the switching regulator is in a light-load driving mode such as a stand-by state and a sleep-mode state, the efficiency of the switching regulator is substantially reduced.
In light of the above, the Japanese Laid-Open Patent Publication No. 2002-300774, for example, discloses a method of reducing the electricity consumed in the switching regulator by switching from a PWM (pulse width modulation) control to a PFM (pulse frequency modulation) control in the light-load driving mode, so that the switching frequency decreases to improve the efficiency of the switching regulator in the light-load driving mode.
An exemplary background switching regulator according to the above disclosure is illustrated in FIG. 1. The switching regulator 100 of FIG. 1 includes a PWM control circuit (PWMC) 101, a PFM control circuit (PFMC) 102, a switching element 103 driven by the PWM control circuit 101, a switching element 104 driven by the PFM control circuit 102, an oscillation circuit (OSC) 105, an error amplifier circuit (EAC) 106, a reference voltage source 107, an inductor La, a diode Da, a capacitor Ca, and resistors Ra and Rb. The switching regulator 100 receives an input voltage from a direct current power source BAT and outputs an output voltage to a load LO.
In normal operation, the PFM control circuit 102 stops operation, and the PWM control circuit 101 operates to perform an ON/OFF control of the switching element 103. Meanwhile, in the light-load driving mode, the PWM control circuit 101 stops operation, and the PFM control circuit 102 operates to perform an ON/OFF control of the switching element 104.
Since a relatively large amount of current flows through the switching element 103, which is used when the PWM control is performed, the switching element 103 is designed to be increased in size to reduce an ON-resistance thereof. The increase in size of the switching element 103, however, also increases a gate capacitance thereof.
When the amount of current supplied to the load (hereinafter referred to as load current) is relatively large, efficiency loss of the switching regulator is mainly due to the ON-resistance of the switching element used in the switching regulator. On the other hand, when the amount of the load current is relatively small, the efficiency loss of the switching regulator is mainly caused by charging and discharging of the gate capacitance of the switching element used in the switching regulator. Therefore, the switching element 104 used under the PFM control is reduced in size to have a relatively large ON-resistance but a relatively small gate capacitance, so that the efficiency of the switching regulator is improved.
However, when the switching regulator is designed to change the output voltage output therefrom such that the output voltage is switched from a relatively low voltage value to a relatively high voltage value according to a voltage setting signal, an overshoot voltage is generated, as observed in FIG. 2. The overshoot voltage causes a change in the output voltage. In addition, the overshoot voltage is increased when a switching regulator having a relatively large ON-resistance is switched to a switching regulator having a relatively small ON-resistance.
Further, the light-load driving mode in which the load current is small in amount is, in many cases, the sleep-mode state or the stand-by state wherein a load circuit such as a CPU (central processing unit), which uses the switching regulator 100 as a power supply circuit thereof, is stopped in operation. In this light-load driving mode, an operating voltage for the load circuit may be reduced to be lower than an operating voltage applied in a normal driving mode. Therefore, it is common that the output voltage output from the switching regulator is lowered to reduce the load current.
When switching is made from the light-load driving mode to the normal driving mode, however, if a control mode of the switching regulator is switched from the PFM control to the PWM control, and at the same time, if the output voltage is changed from a relatively low voltage to a relatively high voltage, a relatively large overshoot voltage is generated, causing adverse affects on the CPU and other circuits.